The present invention relates to a new process for the production of water-blown polyurethane foams, from starting materials known per se, using special .alpha.,.beta.-unsaturated ester carboxylic acids in the form of alkali metal salts as catalysts. More particularly, the invention relates to the production of composites by back-foaming of plastic films with reaction materials containing such catalysts which react to form polyurethane foam, and to the composites obtained therefrom.
Water-blown foams, more particularly composite films, produced by back-foaming of a plastics film with a reaction mixture that reacts out to form a flexible or semirigid polyurethane foam are of considerable industrial significance. These are widely used, for example, for the interior trim of motor vehicles and aircraft, and for the production of upholstery. For most of these applications, it is crucial that the foam have very little influence, if any, on the thermal ageing of the surface layer, in addition to having good mechanical properties.
More particularly, the thermal ageing behavior of known back-filled foams (i.e. the tendency of the PVC surface layer to turn brittle and crack) is a problem worth solving.
Constituents of the reaction mixture which react to form the polyurethane foam which promote poor thermal stability are, for example, the tertiary amines often used as catalysts. Previous attempts have been made to replace these amines, for example, by alkali metal salts of carboxylic acids, such as potassium acetate for example. However, catalysis with these catalysts alone is attended by the disadvantage of an excessively long demolding time or, where the catalyst is used in high concentrations, an excessively short cream time. Although the use of organotin compounds generally leads to an improvement in ageing behavior, it also leads to problems concerning the constant activation of the polyol component over a prolonged storage period on account of the hydrolysis stability of the organotin compounds in the water-containing polyol. In addition, so-called amine trapping agents are added to the foam system to improve its thermal stability. Unfortunately, these lead to only a gradual improvement and are unsuitable for amine-catalyzed systems.
U.S. Pat. No. 4,868,043 recommends the use of ester carboxylic acids as catalysts. These ester carboxylic acids are present in the alkali metal salt form of (i) intramolecular carboxylic anhydrides and (ii) monohydric alcohols free from ether groups. In particular, the use of semiesters present in the alkali metal salt form of (i) intramolecular carboxylic anhydrides and (ii) long-chain diols is preferred. However, the last of the above-mentioned catalysts, which are preferably used according to the document in question, have the disadvantage that, due to their monofunctionality in the context of the isocyanate addition reaction, they result in chain termination reactions and, accordingly, frequently result in poor processability of the reaction mixture and in foams of reduced tear strength. The ester carboxylic acid salts free from hydroxyl and ether groups which are not among the preferred catalysts according to the document cited above, show unsatisfactory compatibility with the other reactants. In addition, both types of catalyst described in U.S. Pat. No. 4,868,043 lead to reaction mixtures which do not optimally satisfy practical requirements with regards to the patio of cream time, which is preferably long, to demold time, which is preferably short.
It has now surprisingly been found that the problems mentioned hereinabove can be solved using the catalysts of the present invention which are described in detail hereinafter. The key point of the process according to the invention is that the catalysts used are selected .alpha.,.beta.-unsaturated ester carboxylates which do not exhibit the disadvantages mentioned hereinabove. This is due to their content of ether groups and/or their comparatively high content of carboxylate groups, coupled with the absence of chain terminating hydroxyl groups. However, these catalysts are capable, due to the presence of olefinic double bonds adjacent to the carboxylate groups, of the addition of amino groups which can form during the gradual ageing of the foam. Accordingly, the ageing of the foam and, optionally, the surface film is distinctly reduced. Although this mechanism of so-called amine trapping agents has hitherto been achieved with additives, the additives in question could not be used as catalysts for the PUR reaction (see also G. Avar, J. Thompson-Colon, SAE Conference 1988, 880 507).
The earlier German Patent Application, DE 4,218,840.7 is concerned with the use of similar catalysts in the production of polyurethane-based composite films. However, the ester carboxylates mentioned therein contain on a statistical average at least two hydroxyl groups per molecule. The present invention is not concerned with the use of such compounds.